Wildlife researchers said Tuesday that they've discovered 125,000 western lowland gorillas deep in the forests of the Republic of Congo, calling it a major increase in the animal's estimated population.

The Wildlife Conservation Society, based at New York's Bronx Zoo, and the Republic of Congo said their census counted the newly discovered gorillas in two areas of the northern part of the country covering 18,000 square miles.

Previous estimates, dating to the 1980s, put the number of western lowland gorillas at less than 100,000. But the animal's numbers were believed to have fallen by at least 50 percent since then due to hunting and disease, researchers said. The newly discovered gorilla population now puts their estimated numbers at between 175,000 to 225,000.

The researchers in the Central African nation of Republic of Congo -- neighbor of the much larger Congo -- worked out the population figures by counting the sleeping "nests" gorillas make. The creatures are too reclusive and shy to count individually.

The report was released as primatologists in Edinburgh, Scotland warned that nearly half of the world's 634 types of primates are in danger of becoming extinct due to human activity. That figure, carried in a comprehensive review of the planet's apes, monkeys, and lemurs, included primate species and subspecies.

Scientists meeting at the International Primatological Society Congress in Edinburgh said they hoped the report will help spur global action to defend mankind's nearest relatives from deforestation and hunting.

Primatologists warned that species from the giant mountain gorillas of Central Africa to the tiny mouse lemurs of Madagascar are on the "Red List" for threatened species maintained by the IUCN.

The review was funded by Conservation International, the Margot Marsh Biodiversity Foundation, Disney's Animal Kingdom and the IUCN. It is part of an examination of the state of the world's mammals due to be released at the 4th IUCN World Conservation Congress in Barcelona, Spain, in October.